1. Field of the Invention
The present invention relates generally to an energy absorber, and, more particularly, to an energy absorber suitable as a support member for bumpers attached to an automobile or a shock absorber that is used under the floor portion of a helicopter.
2. Description of the Related Art
Generally, a pair of bumpers are attached to the front and rear of the body of an automobile, respectively. The bumpers absorb the impact energy at the time of collision to protect the body of the automobile and the occupants. Bumpers are required to irreversibly absorb an extensive load that would be applied to an automobile when the automobile collides with an obstruction. To increase the absorbing energy of the bumpers, there have been made various improvements on the materials and structures of support members that support the bumper bodies.
A shock absorber is used under the floor portion of a helicopter. This shock absorber serves to reduce the impact as much as possible if the helicopter should make an emergency landing due to some malfunction or the like, and particularly to reduce the shock the passengers would receive. There is a demand for shock absorbers that are light and exhibit excellent energy absorbing performance.
For example, German Laid Open Patent Publication No. DE 3,626,150 A1 laid open on Feb. 18, 1988 discloses bumpers attached to the stay of the body of a vehicle through an elastic attenuating member, which is made of fiber reinforced plastic. The attenuating member is formed in substantially ring shape, with the fibers of the fiber reinforced plastic arranged in the circumferential direction. The attenuating member is used in a state where the impact is applied thereto from the side or where the axis of the attenuating member is perpendicular to the direction to which the impact is applied.
When destructive force is externally applied from the side to the substantially ring-shaped member of fiber reinforced plastic as disclosed in the German patent publication, however, only the portion which extends in the same direction as the load-applying direction is deformed to be broken. In other words, that portion which extends in the direction perpendicular to the external force will substantially retain the original shape and will not be broken. When a load is applied to this ring member, therefore, this member absorbs a small amount of energy during the compressive deformation and has a poor energy absorbing efficiency per weight of the member.
Japanese Unexamined Patent Publication No. 124142/1982 proposed a cylindrical net article 22 formed of a fiber composite tape 21 as shown in FIG. 11, as a shock protective article that is used for bumpers. For example, the tape 21 is made of a material that has a glass fiber roving impregnated with an epoxy resin. The tape 21 is arranged with an inclination of 30 to 60 degrees to the lengthwise axis of the article 22. Each node 23 of the article 22 is formed of about ten layers of the tape 21.
This shock protective article serves its purpose while supporting the associated bumper in such a way that the compression load is applied from the axial direction of the article 22. When the axial directional load is applied to the article 22, interlaminer separation occurs at the opposing nodes 23 of the net structure, resulting in shear yield at the interface between the fibers and matrix. Accordingly, the energy is absorbed gradually. When the article 22 is broken by the compression load, the breakage occurs everywhere. Thus, the energy absorbing efficiency per weight of the absorber in this case can be enhanced compared with the case where a compression load is applied from the side. Since this shock protective article has a net structure with an angle of intersection of the tape 21 of 30 to 60 degrees, however, when a compression load is applied in the axial direction, the net structure deforms so that the article 22 easily deforms with a small load.
In addition, taking the requirement of bumper support members to reduce impact on a human body into consideration, the maximum value of such a load should be suppressed down to a level where the human body will not be seriously affected. The amount of energy absorption is expressed specifically by the area under the curve in the graph which shows the relationship between the compression load and the amount of displacement, and above the base scale which represents the amount of displacement. When there is an excessive change in load, the total amount of energy absorption becomes smaller. To meet the requirement of reducing impact on the human body while increasing the amount of energy absorption at the time of deformation, the following two points are important.
(1) To prevent unexpected generation of a load.
(2) To keep the curve of the compression load vs. the amount of displacement as flat as possible (i.e., to make a change in load as small as possible).
As the load gradually decreases with an increase in the amount of displacement for the above shock protective article, however, it is difficult to increase the amount of energy absorption.
Recently, so-called air bags are employed to protect occupants in an automobile. The air bags are designed to expand to protect occupants, when the load that is generated upon collision is detected to be equal to or above the level which is dangerous to human bodies. It is rather dangerous if the air bags are activated upon low-speed collision that will not cause serious damage on the occupants. It is therefore necessary to prevent oversensitive activation of the air bags. The activation sensor of each air bag is designed not to function with a load equal to or below a predetermined level, but to function only when the load reaches a second level higher than the predetermined level. As a tuner for the activation sensor to activate the air bag under the desired conditions when an automobile collides at a predetermined speed or higher, it is therefore necessary to provide an energy absorber which breaks while generating predetermined loads associated with two levels of collision speeds. However, there has been no proposal so far to design an energy absorber in such a way as to generate two levels of load change in association with two levels of collision speeds so that it can serve as a tuner for the activation sensor of an air bag.
Further, as there are many possible speeds and directions of collision of an automobile, the energy absorber should cope with many levels of breakage load accordingly.